Li-ion batteries dominate the mobile device market due their high energy density; and their application is being extended to the automotive and grid storage industries. However, charge storage limitations limit the applicability of classic Li-ion technologies into these new markets, thus triggering interest in new energy storage concepts. Multivalent ion intercalation is an attractive alternative because, while conceptually similar to Li-ion, they store more charge per intercalated species and observe less dramatic structural rearrangement in the host. Recently, reversible electrochemical Mg intercalation into spinel-type Mn₂O₄ has been reported, where Mg²⁺ ions occupy vacant tetrahedral sites [1]. First principles calculations by Lui et al. argue that other multivalent ions (i.e. Ca²⁺, Zn²⁺, Al³⁺, Y³⁺) may be intercalated into spinel host framework to yield high voltage cathode materials for multivalent batteries [2].

In this work, cubic phase Mn₂O₄ was prepared electrochemically by delithiating commercially available spinel-type LiMn₂O₄ in a non-aqueous environment. Following, Mn₂O₄electrodes were reduced in concentrated aqueous solutions of Ca-, Mg- and Zn nitrate to -0.4 V/SCE. As-prepared electrodes were characterized using powder x-ray diffraction. As shown in the figure, the cubic structure is preserved upon delithiation and significant structural changes occurred upon reduction; indicating the formation of a tetragonal spinel (likely tetrahedral site occupation) in addition to the parent cubic phase.  X-ray absorption spectroscopy and atomic resolution microscopic methods will also be used to help understand the structural changes as these divalent cations are intercalated into a spinel host framework.

References:

1. Kim, C., et al., Adv. Mater. 2015, 27, 3377
2. Miao, L., et al., Energy Environ. Sci. 2015, 8, 964